1. Field of the Invention
The present invention relates generally to optical power control in a passive optical network (PON) and, more specifically, to adjusting a burst-mode data transmission preamble in response to power measurement.
2. Description of the Related Art
Most digital telecommunications networks (i.e., networks that facilitate the communication of data, voice, video, etc., between parties or between a content distribution service and subscribers) typically comprise active components, such as repeaters, relays and other such devices that consume power, in the path between an exchange and a subscriber. In addition to requiring power, active components are subject to failure and performance degradation over time, and may require significant periodic maintenance. The passive optical network (PON) has been developed to overcome some of these deficiencies. The essence of a PON is that nothing but optical fiber and passive components are found in the path between the exchange and subscribers. A single fiber can run from the exchange to a passive splitter located near a group of subscribers, such as a neighborhood or office complex, and individual fibers can run from the splitter to individual subscribers or sub-groups of subscribers.
The International Telecommunications Union (ITU) and the Institute of Electrical and Electronics Engineers (IEEE) are two standards-making bodies currently developing PON standards. The ITU has adopted recommendations of the Full Service Access Networks (FSAN) organization, including G983.x, a specification for broadband PON (BPON), also referred to as “APON,” a reference to the Asynchronous Transfer Mode (ATM) data transmission protocol, and G984.x, a specification for gigabit PON (GPON). These standards and recommendations are well-known to persons skilled in the art to which the invention relates and are therefore not described in further detail herein (i.e., in this patent specification).
In accordance with these standards and recommendations, a PON comprises an optical line terminator (OLT) at the exchange or central office and a number of optical network units (ONUs), also known as optical network terminals (ONTs), each located at or near the subscriber's premises (e.g., home, office building, etc.), with optical fiber and splitters between the OLT and ONUs. In the downstream direction, i.e., data transmitted from the exchange to a subscriber, the data units (e.g., ATM cells, packets, etc.) are broadcast from the OLT to all of the ONUs in the network, and an ONU can select the data to receive by matching the address embedded in the data units to a selected address. In the upstream direction, i.e., data transmitted from a subscriber to the exchange, the data units are time-division multiplexed with those transmitted from other subscribers. BPON and GPON are sometimes referred to as burst-mode PON technologies because they transmit bursts of data packets at relatively high bit rates.
Power control is an important consideration in burst-mode optical networks because it can help minimize data transmission errors. In a PON, bit errors can occur if the amplitude of upstream data packet bits received at the OLT is outside the OLT receiver operating range. In other words, it is important that the signal be neither too powerful for the OLT receiver and thus overload it nor too weak for the OLT receiver and thus become obscured by noise. The amplitude of the upstream signal received at the OLT can vary from ONU to ONU for a number of reasons, including the number of splits in the paths and the different distances from the OLT at which the ONUs may be located.
One power control mechanism set forth in the G984 specifications, known as “Power Leveling,” involves the OLT measuring the average amplitude of a packet received from an ONU and, if the amplitude is outside the OLT receiver sensitivity range, transmitting a command to that ONU that causes it to adjust its transmission power upwards or downwards.
The G984 specifications also provide for the OLT to account for differences in amplitude by “training” its receiver to each upstream packet, i.e., adjusting itself to the amplitude range of that packet, in order to receive the packet data without errors. The G984 specifications provide for inclusion of a preamble preceding the data bits of each packet to use in training the OLT receiver. The required training time depends largely upon the amplitude difference between consecutive upstream packets. That is, a very bright packet received at a high optical power level from an ONU close to the OLT followed by a dimmer packet received at a lower optical power level from an ONU farther from the OLT creates a difficult situation for the OLT receiver, requiring a long recovery time before being able to receive the next packet. The greater the amplitude difference between such consecutive packets, the longer the training time that is needed, and thus the longer the preamble that is needed. Conventional systems are designed to use a preamble of a predetermined or fixed length that is long enough to accommodate the largest (i.e., worst-case) amplitude difference that is specified by the ITU or IEEE standards. An amplitude difference of 15 dB is typically used as this worst-case difference. (See ITU G984 standard.) A preamble long enough to train an OLT receiver in the case of such a worst-case amplitude difference can consume up to 10 percent of the available upstream bandwidth.
However, in PONs likely to be used commercially, the amplitudes of signals received at the OLT are unlikely to vary much from ONU to ONU—much less than the worst-case 15 dB—because ONUs are generally located at about the same distance from the OLT. Thus, the worst-case amplitude difference is believed to be very conservative for most commercially viable PONs. It would be desirable to provide a method and system that maximizes upstream data transmission bandwidth without compromising bit error rate. The present invention addresses these problems and deficiencies and others in the manner described below.